Fix metadata for create in child cell

Metadata can be passed as a list from parent cell. Make sure to convert
it to a dict before calling instance_create() in the child cell.

This also adjusts the previous similar fix for system_metadata to make
sure to not drop any sys_meta that was passed down.

Fixes bug 1197166

Change-Id: I38937ebe5ae81158d51992f4efd047531b53302c
Merge "Fix metadata for create in child cell"